Q:
In a book I recently read, the author alleges that cosmic rays are made of high-energy
photons which collide with each other to form
particles, and so all the particles are "frozen light"
because they are formed when photons slows down.
My understanding is that cosmic rays are made of
protons and atomic nuclei which collide and form other
particles, giving off some photons in the process, and
that photon does not slow down to become anything
else (the decrease in the frequency of a photon is
not the same as the decrease in speed) and its
existence as far as is now known is independent of other
bosons and leptons. Can you confirm either notion?

A:
Photons travel at the speed of light,
this is almost always true (there are interesting experiments taking
place at Harvard and other places which have been able to slow light
down, but in nature we only see photons traveling at the speed of
light). If there were a photon-photon collision in the atmosphere, it
would happen at the speed of light. Any particles resulting from the
collision would be very short lived, and I believe they are not the
cosmic rays that we observe.

Cosmic rays are mostly high-energy protons and helium nuclei, as you
mention. Part of the cosmic rays originate from the Sun, generated by
solar flare events. We believe the remainder come from interstellar
space, likely from supernova and perhaps from even larger explosions, a
star collapsing into a black hole for example. We're actually not sure
of the origin of much of the cosmic ray flux. There are many different
astrophysical processes that could send high-energy particles flying
out and we're not sure which ones contribute how much to the cosmic ray
flux we observe.